1. Field of the Invention
The present invention relates to a method of heat treating stainless steel.
2. Description of the Related Art
European Patent Specification No. 0 804 622 teaches a method of heat treating stainless steel that is highly beneficial with respect to conventional techniques.
That earlier specification relates to the heat treatment of stainless steel in the form of piping, tubing, strip material, or wire material, such as strip material, sheet, wire, or rods, after rolling the material, and at a temperature higher than about 900° C. in a furnace or oven. According to that invention, the burners provided in the furnace are driven with a liquid or gaseous fuel which is caused to combust with the aid of a gas that contains at least 85% by volume oxygen and at maximum 10% by volume nitrogen.
This known invention generates during the combustion process a flue gas that contains essentially water and carbon dioxide. The amount of heat that radiates from the flue gas to said material is much greater than the heat radiated from a flue gas that is the product of a combustion process in which air is used as an oxidant. The heat transferred by radiation is the dominant heat transfer in such a heat treatment process.
This greater transfer of heat significantly shortens the time taken to heat the material in the furnace, therewith enabling the rate at which said material is fed into a given furnace to be greatly increased.
Moreover, it was found that the oxide scale formed on the material surface when heated is thinner and can be dealt with more easily, owing to the fact that the scale has a structure which differs from the structure of the scale that forms when the material is heated in a furnace with a conventional air-based flue gas. The pickling time, i.e., the residence time of the material in a following acid and/or electrolyte bath, can be shortened due to the thinner oxide scale.
With the method according to that patent specification a thin oxide scale is thus formed, which remains relatively bright on the furnace.
One drawback with that method is that the comparatively bright surface of the material counteracts the high heat transfer that could otherwise be achieved by the radiation components in an oxygen-gas-based furnace atmosphere. When practicing the method according to that prior publication, the heat transferred is of the order of 30-150 kW/m2.
The present invention solves this problem and enhances the transfer of heat by radiation.